1. Field of the Invention
The present invention relates to semiconductor lithography.
2. Prior art
Considerable effort over the past two decades has been expended to develop techniques to fabricate materials with ultrasmall dimensions, usually embedded in a protective or interactive matrix. The constraints on size are highly dependent upon the specific application, however the ultimate goal is to produce materials that can be engineered from macroscopic dimensions to atomic proportions in size. Attempts at small device fabrication of solid materials have taken two general approaches. The first utilizes a macroscopic deposition of materials followed by removal of excess material on a microscopic scale, and the second involves a variety of specialized growth techniques, such as molecular beam epitaxy and metallo-organic chemical vapor deposition (MOCVD), where the "device" is incorporated into the structure during the growth process. Although great progress has been made, a host of problems still need to be resolved. There is no good way to manufacture highly uniform and symmetric arrays containing greater than 10.sup.3 elements of quantum confined materials. Further, if the elements are quantum confined optical devices, they have the additional constraint that their performance is especially susceptible to material damage.
The semiconductor industry relies on a variety of lithographic mask types to define regions where deposition, or lack of deposition, of dissimilar materials will occur. Electron and ion beam direct writing on energy sensitive surfaces have been developed to produce a mask capable of defining structures as small as 10 nm. To obtain high resolution, the beams are tightly focused which requires a large acceleration of the charged particles composing the beam. After the particles pass through the lithographic medium defining a pattern, they pass into the substrate and stop, where they deposit their remaining kinetic energy. The interaction of the particles with the substrate usually causes damage to the substrate, which in most cases is detrimental to small device fabrication. In addition, scattering of the particles in the substrate back into the lithographic material usually reduces the sharpness and contrast of the mask, lowering the mask resolution.
Another type of lithographic mask involves photoresistive material. Photons pass through the transparent portions of an otherwise opaque mask and reproduce the pattern on photoresistive material, forming a resist mask. Resolution of the pattern on the resist is ultimately limited by the wavelength of the photons used to form it (i.e. minimum resolution&gt;wavelength/2). A pattern with structures 10 nm in width and a 50% variation in definition would require photons with wavelength less than 10 nm, a demanding region of the spectrum in which to work.
Once the resist mask is fabricated, it is possible to add or remove materials defined by the mask. Addition of materials such as metals generally preserves the resolution of the mask. However, removal of substrate materials below the mask requires techniques such as reactive ion etching (RIE), chemical etching, etc. These processes usually distort the image of the mask as the etching process continues deeper into the substrate. In addition, the interaction of the RIE particles with the substrate usually causes damage to the substrate, which in most cases is detrimental to small device fabrication.
A third type of mask is one where deposition of desired material is physically blocked or passed in regions by prefabricated mask laid down on the substrate. To date the smallest structures within these masks are on the order of 10's of microns in size.
Devices that utilize tunneling of electrons or elementary excitations such as excitons, are very sensitive to barrier heights and widths. Arrays of devices that require identical barriers (i.e. identical device characteristics) then become extremely sensitive to the overall composition of the array in the elements within the array. Variations of less than 10 percent in size or position of the quantum confined array element are sufficient to wash out the optical signature of both cooperative effects and energy shifts due to confinement.
Recent attempts at defining quantum confined arrays of wires or dots have relied on a variety of masking, etching and growth techniques. Due to the small size requirements for quantum confinement (often &lt;50 nm), these processes usually contaminate the quantum material by damage or extraneous material deposition, which is generally detrimental to device performance. Where material damage is intentionally invoked to define the structure of the device, component boundaries tend to be vague, compromising large scale geometries. Arrays fabricated by controlled growth techniques over substrates tend to be extremely labor and time intensive, and exhibit good uniformity over a relatively small region of the array. Chemical etching of free standing arrays utilizing masking techniques tend to be fragile and curve definition is extremely process and material dependent.